Extension across Valles Marineris and the Thaumasia “rift”, Mars

We calculate the amount of crustal stretching across the Valles Marineris and a large graben structure in the western Thaumasia region (the “Thaumasia graben” [1,2]). We measure vertical offsets in a set of transects using data obtained by the MOLA laser altimeter. Assuming a fault dip angle of 60°, correcting for landslide deposits, but ignoring other sediment cover, we obtain extension values from 9 km in western Ius Chasma to 20 km in eastern Melas Chasma, decreasing toward eastern Coprates Chasma. The Thaumasia graben in the Claritas Fossae region shows extension of 0.5 to 4.5 km. An interpretation and discussion of the results will be given elsewhere. Method: A set of profiles was analyzed for each area using a Digital Elevation Model (DEM) generated with data from the MOLA instrument. The MOLA data set [3] has a higher accuracy than previous DEMs computed from Viking stereo imagery, occultation data, and Earth-based radar measurements. It is also more accurate than height information from photoclinometry or shadow measurements. Drawbacks of MOLA data are the relatively low lateral resolution (~330 m between data points along track, i.e., in a roughly N-S direction, much wider spacing between data across track, i.e., in E-W direction), as compared to available image data. It is, therefore, often ambiguous whether a topographic offset is caused by faults or by other morphologic features, e.g., by impact craters. This problem can partly be overcome by merging MOLA-based DEMs with Viking or MOC image data (Fig. 1 and 2). The extension e was calculated by: